1. Field of the Invention
The present invention relates to an apparatus of detecting mismatch between an in-phase-channel (I-channel) signal and a quadrature-channel (Q-channel) signal in an orthogonal frequency division multiplexing (OFDM) receiver, and more particularly, to a time-domain IQ mismatch detection apparatus in an OFDM receiver capable of minimizing an influence of fading caused from a channel.
2. Description of the Related Art
An orthogonal frequency division multiplexing (OFDM) scheme is a well-known high-speed data transmission scheme for the next-generation communication technology. In the OFDM transmission scheme, a sequence of symbols which are serially input is converted into parallel data (parallel symbols) in units of a predetermined block, and a plurality of the parallel symbols are multiplexed with different subcarrier frequencies. The OFDM scheme is implemented by fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT). The OFDM scheme can be simply performed by using the orthogonality between the carriers and the definition of the FFT.
In a radio frequency (RF) stage which converts a RF signal into a baseband signal, cost of an RF processing structure and system complexity are increased. In order to solve the problems, a zero-IF scheme for directing converting the RF signal into the baseband signal without use of an intermediate frequency (IF). In addition, the zero-IF reception structure can be implemented as the RF stage into a system-on-chip (SoC) structure which is not complicated.
However, an actual zero-IF receiver has shortcomings in that it is impossible to completely perform image rejection in the down-converting process using orthogonal demodulation.
The shortcomings are caused from incompleteness of analog circuits such as incompleteness of 90°-shifting of a local oscillating signal generated by a local oscillator and mismatching of a mixer and a filter. The impossibility of image rejection results in IQ mismatch between an I-channel signal and a Q-channel signal of a baseband receiver.
As an approach for solving the IQ mismatch, high-performance analog parts having less than 2° of phase mismatch and less than 2% of amplitude mismatch can be used to reject the image. However, since these analog parts are very expensive, product cost is highly increased. Therefore, there have been proposed approaches for compensating for the IQ mismatch of signals received at a baseband by using inexpensive analog parts.
Conventional IQ mismatch compensating schemes are mainly classified into a frequency-domain IQ mismatch compensating scheme and a time-domain IQ mismatch compensating scheme.
As a reprehensive scheme of the frequency-domain IQ mismatch compensating scheme, there is a scheme in where a transmitter transmits a specific pilot signal, and a receiver receives the pilot signal to estimate IQ mismatch. In the frequency-domain IQ mismatch compensating scheme, signal deformation caused from a channel and signal deformation caused from IQ mismatch can be compensated simultaneously, so that an effective IQ mismatch compensation performance can be obtained. However, the frequency—domain IQ mismatch compensating scheme can be applied to only the signal having the pilot signals in a predetermined time interval. In a system such as a terrestrial digital multimedia broadcasting (T-DMB) system with a signal structure having a small number of the pilot signals (for example, a signal structure having 76 symbols in one frame wherein the first symbol of one frame being the pilot signal), the IQ mismatch compensation performance is deteriorated. Therefore, the frequency-domain IQ mismatch compensating scheme is not suitable for a receiver of the T-DMB system.
On the other hand, in a conventional time-domain IQ mismatch compensating scheme, the pilot signal is not required. FIG. 1 is a block diagram illustrating a construction of an apparatus for implementing the conventional time-domain IQ mismatch compensating scheme (hereinafter, simply referred to as a time-domain IQ mismatch compensating apparatus).
Referring to FIG. 1, the conventional time-domain IQ mismatch compensating apparatus includes: a correlation compensator 11 which compensates for amplitude and phase of an I-channel signal sIadc (n) and a Q-channel signal sQadc (n) input according to output signals ua(n) and up(n) of a first loop filter 121 and a second loop filter 131; a amplitude mismatch detection unit 12 including an amplitude mismatch detector 121 which detects an amplitude difference between IQ-mismatch-compensated I-channel signal sI(n) and Q-channel signal sQ(n) output from the correlation compensator 11 and a first loop filter 122 which filters an amplitude difference signal ea(n) detected by the amplitude mismatch detector 121 so as not to be diverged; and phase mismatch detection unit 13 including a phase mismatch detector 131 which detects a phase difference between the IQ-mismatch-compensated I-channel signal sI(n) and Q-channel signal sQ(n) output from the correlation compensator 11 and a second loop filter 132 which filters an phase difference signal ep(n) detected by the phase mismatch detector 131 so as not to be diverged. The amplitude mismatch detector 121 subtracts an absolute value of the Q-channel signal from an absolute value of the I-channel signal to obtain the amplitude difference, and the phase difference mismatch detector 131 multiplies the I-channel signal with the Q-channel signal to obtain the phase difference.
According to the time-domain IQ mismatch compensating scheme implemented by the time-domain IQ mismatch compensating apparatus as shown in FIG. 1, the amplitude mismatch and the phase mismatch are detected in units of a sample included in a symbol of the OFDM signal.
As described above, the time-domain IQ mismatch compensating scheme can be applied to compensate for the IQ mismatch in a case where there is no pilot signal. However, in a case where the received OFDM signal is influenced by fading caused from a communication channel, reliabilities of the amplitude difference detection and the phase difference detection of the amplitude mismatch detector 121 and the phase mismatch detector 131 may be deteriorated.
Therefore, in a system such as a T-DMB system having a zero-IF type RF processing structure, an apparatus for detecting the amplitude mismatch and the phase mismatch of IQ signals capable of minimizing an influence of fading by using a time-domain IQ mismatch compensating scheme without use of a pilot signal is required.